Yes, we have seen this ourselves and looked into it. It turns out that it is a bug in macOS 10.12 Sierra. When macOS 10.12 saved the image, it didn't save *enough* information. But also in 10.12, it didn't *look* for that information so no one noticed, including Apple. Now, in macOS 10.13, it's looking for that information but it's missing. So the image looks washed out.

Right now, you will only see a problem if you created the picture in 10.12 and are looking at it in 10.13. We have contacted Apple to find out what the right way to fix it is, but they have not yet said. It has happened to many other applications besides ours.

In the mean time, I would do nothing with your image yet and just wait to see what the fix is. We need to make a fix to how we save the image when running under 10.12. But also, it's likely that we can do something on 10.13 to "fix" the image so that it looks fine again, by just adding the missing information to existing pictures. We just want to make sure that we do it correctly so are waiting for some insight and detailed information from Apple.

Alternatively, if you have upgraded your Mac to 10.13 and still have the original image and can easily replace it (for example, if the image link is still valid), then you can do that and just save it again. Saving it under 10.13 should be fine.